This Forbes article explains how it got onto Steam in the first place.

Many people are criticizing Valve for what is obviously an unfinished product showing up in their store. The problem appears to be a simple one. Because the game’s publisher had a game on Steam previously, their new titles can skip the review process, giving The War Z a free pass through the back door.

Valve may need to reconsider this policy in light of War Z‘s numerous problems, especially since many gamers will certainly demand a refund.